Various tread patterns are employed in a tire tread according to conditions of a vehicle and road surface. For a tread pattern, block, rib, lug and other patterns comprising units of a certain pattern, that is, pattern composing units repetitively provided in the circumferential direction of tire, are often used by circumferentially forming grooves in the direction of tire's axis with a spacing between them, or providing rib grooves circumferentially staggered.
In a tire with such patterns repetitively provided, a noise is repetitively produced between the tire and a road surface, because the pattern composing units in an alignment (sequence) come in contact with the road surface sequentially as the tire travels. The noise produced by the pattern composing units (hereinafter referred to as pitch noise) is generally uncomfortable, and improvement of the noise is desired.
In order to improve the noise, the so-called pitch variation method has been conventionally employed for dispersing the noise in a wide frequency band, and changing it to a white noise by arranging pattern composing units different in pitch or length in the circumferential direction of tire.
For changing it to a white noise, a lot of suggestions have been made, and provision of pitches of a periodic sequence in a manner of sinusoidal function is suggested, for example, in Japanese Patent Publication Nos. 58-2844 (Japanese Laid-open Patent Nos. 55-8904) and 3-23366 (54-115801). It is also suggested to provide a random sequence of pitches of pattern composing units as described in Japanese Patent Publication No. 62-41122 (57-114706).
It is suggested to provide s sequence of two pitches of two pattern composing units (the number s=2) as described in Japanese Patent Publication Nos. 51-41723 (Japanese Laid-open Patent No. 50-20402) and Japanese Laid-open Patent No. 4-363234.